The present invention relates generally to a projection system for printing circuit patterns on photoresist coated wafers in semiconductor microlithography. It relates in particular to a class of projection systems wherein a photoresist coated silicon wafer is covered with circuit patterns by sequentially exposing different areas of the wafer. Such a system is generally referred to as a step-and-repeat exposure system or simply a stepper.
The present invention relates more particularly to an alignment system for a unit magnification optical projection system for use in microlithography such as described in U.S. Pat. No. 4,964,705 which is assigned to the same assignee as the present invention, and the details of which are hereby incorporated by reference.
That unit magnification projection system is a unit magnification, achromatic, anastigmatic, optical projection system that uses both reflective and refractive elements in a complementary fashion, i.e, a catadioptric optical system, to achieve large field sizes and high numerical apertures. In that system the reticle and wafer are parallel with a window being provided on the reticle adjacent to the patterned area to allow for the projection of the reticle (mask) pattern through the reticle window onto the wafer. The unit magnification system is referred to as a Half-Field Dyson system for reasons which will be explained in detail in the present specification.
Semiconductor wafer fabrication generally involves several stages of fabrication. A fabrication stage may include patterning a photoresist coated metalized wafer using a projection apparatus, developing the patterned resist to form a photoresist mask, and etching a pattern in the metal through the mask. Fabrication stages are carried out sequentially until a complete circuit having several layers of metallization or other circuit material is built up. At each fabrication stage a wafer must be exactly re-positioned or aligned with a reticle providing a layer pattern in such a way that all layers on the wafer will have the correct registration with each other. To facilitate correct alignment, a reticle may be provided with a set of one or more alignment keys, while a wafer may be provided with a corresponding set of one or more alignment targets. The projection apparatus may be provided with means for observing the positional relationship of the set of reticle keys with the corresponding set of wafer targets, and generating data which may be used to move the reticle and wafer with respect to one another to align the wafer targets with a projected image of the reticle keys. The wafer and reticle may thus be aligned such that a pattern on the reticle will be projected onto the wafer in correct alignment with other patterns which may be already on the wafer.
During the wafer fabrication process, alignment targets may be covered with photoresist material. Consequently, alignment for a projection system preferably is done using wavelengths at which the photoresist is transparent. A Half-Field Dyson projection system is illuminated, for projection purposes at a wavelength of about 248 nanometers (nm). UV photoresist materials are generally absorbing at 248 nm. An alignment system preferably is illuminated with a broad band of wavelengths, longer than 248 nm, to which the photoresist is transparent. Depending on fabrication steps and resists, it may be preferable to illuminate an alignment system with different bands of wavelengths at different stages of fabrication. Multiple wavelength capability is thus an important requirement for an alignment system to be used with a unit magnification projection system such as the Half-Field Dyson System.